1. Field of the Invention
The present invention relates to an exhaust emission control system having a generator for converting thermal energy of exhaust into electric energy, and an exhaust purifying catalyst.
2. Related Background Art
Exhaust gas from an automobile engine possesses thermal energy and discharge of exhaust gas results in wasting energy. There is thus an electric generator configured to convert the thermal energy of exhaust gas into electric energy to charge a battery. An example of such generator is disclosed in Japanese Patent Application Laid-Open No. 2000-35824.
This generator has a thermoelectric conversion element provided on an exhaust pipe, and a bimetallic element as a thermal connection means for conducting heat from the exhaust pipe to the thermoelectric conversion element. While the temperature of the exothermic member (exhaust pipe) is below an allowable temperature limit of the thermoelectric conversion element, the bimetallic element stays undeformed to conduct the heat of the exothermic member (exhaust pipe) to the thermoelectric conversion element. Conversely, while the temperature of the exothermic member exceeds the allowable temperature limit, the bimetallic element is deformed to break the mechanical contact between the exothermic member and the thermoelectric conversion element so as to interrupt the thermal conduction.
The recent automobiles are provided with an exhaust purifying catalyst on the exhaust pipe, in order to eliminate carbon oxide and others from exhaust gas. Its purifying function of this catalyst is exerted at temperatures of more than its activation temperature. It can be contemplated to construct an exhaust emission control system in a configuration wherein the generator as described above is mounted on the automobile exhaust pipe and wherein the catalyst is provided downstream thereof.
In the exhaust emission control system with the generator and the catalyst in this configuration, however, the generator performs the generating operation even during periods in which the catalyst is below the activation temperature, e.g., right after starting an engine. If the heat in exhaust gas is used for generation of electricity, there will arise a problem that the time to arrival of the catalyst at the activation temperature becomes longer, so as to delay the exertion of the purifying action.